Add Healthchecks.io notify hook (#7282)

* Add Healthchecks.io notify hook

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* notify/healthchecks: require an exact "OK" response

hc-ping.com answers HTTP 200 "OK (not found)" for an unknown UUID, so the
prefix match reported success for a mistyped or deleted check, including
the --set-notify test ping. Only treat an exact "OK" body as success.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

notify/healthchecks.sh

@@ -0,0 +1,53 @@
+#!/usr/bin/env sh
+
+#Support Healthchecks.io (hosted or self-hosted)
+#https://healthchecks.io/docs/http_api/
+
+#Required:
+#HEALTHCHECKS_URL="https://hc-ping.com/your-uuid"
+
+#Use with --notify-level 3, so a ping is sent on every cron run, even when
+#all certs are skipped. Otherwise Healthchecks reports the check as down.
+
+healthchecks_send() {
+  _subject="$1"
+  _content="$2"
+  _statusCode="$3" #0: success, 1: error 2($RENEW_SKIP): skipped
+  _debug "_subject" "$_subject"
+  _debug "_content" "$_content"
+  _debug "_statusCode" "$_statusCode"
+
+  HEALTHCHECKS_URL="${HEALTHCHECKS_URL:-$(_readaccountconf_mutable HEALTHCHECKS_URL)}"
+  if [ -z "$HEALTHCHECKS_URL" ]; then
+    HEALTHCHECKS_URL=""
+    _err "You didn't specify the Healthchecks.io ping url HEALTHCHECKS_URL yet."
+    _err "Example: export HEALTHCHECKS_URL=\"https://hc-ping.com/your-uuid\""
+    return 1
+  fi
+  _saveaccountconf_mutable HEALTHCHECKS_URL "$HEALTHCHECKS_URL"
+
+  _hc_url="${HEALTHCHECKS_URL%/}"
+  case "$_statusCode" in
+  0 | 2) ;;
+  1)
+    _hc_url="$_hc_url/fail"
+    ;;
+  *)
+    _hc_url="$_hc_url/log"
+    ;;
+  esac
+
+  _data="$_subject
+
+$_content"
+
+  response="$(_post "$_data" "$_hc_url" "" "POST" "text/plain")"
+
+  if [ "$?" = "0" ] && [ "$response" = "OK" ]; then
+    _info "healthchecks ping success."
+    return 0
+  fi
+  _err "healthchecks ping error."
+  _err "$response"
+  return 1
+}